Depending on a specific composition, lanthanide oxides-transition metal oxides based perovskites AA'A"BB'B"O.sub.3 may exhibit a wide range of technologically important physical properties, such as ferroelectricity, superconductivity, ionic conductivity, oxygen permeability, high catalytic activity. In any particular application of a given perovskite composition exhibiting specific characteristics, well defined powder morphology and homogeneity are required. For example perovskite powders may be suitable as electrocatalysts for oxygen reduction in fuel cells or batteries, for which very fine high specific surface area powders are required.
Multimetal oxides structurally similar to naturally occurring mineral perovskite (CaTiO.sub.3) i.e. AA'A"B'B"O.sub.3, have for decades been studied for their many interesting physical properties, i.e. diamagnetism, ferromagnetism, ferroelectricity, ion conductivity, oxygen permeability, superconductivity, and catalytic activity. Some of them have found a wide industrial application. In particular, those containing lanthanide and transition metal oxides with the formula La.sub.1-x Sr.sub.x M.sub.y M'.sub.1-y O.sub.3 where M and M' is a transition metal, have attracted attention as catalysts, high temperature fuel cell interconnect materials, or oxygen permeable membranes.
The simplest crudest preparation method, often referred to as ceramic preparation method, consists of mixing the precursor solids, mostly oxides and/or carbonates, and calcining the precursor mixture. Such precursor mixture, even when well homogenized by milling, requires high calcination temperatures often in excess of 1000.degree. C. and long reaction times, as well as considerable intermittent grinding to obtain fine powders. This process produces sintered powders of very low surface area, for many applications insufficient phase and particle size homogeneity, and may introduce impurities by long grinding. Consequently the quality of such powders may be detrimental to obtaining specific required properties. For example to maximize the catalytic activity by increasing specific surface area (SSA), various more complex preparation methods have been proposed, evaluated [4-8], and patented. Freeze-drying, introduced as an excellent laboratory technique for catalyst preparation more than twenty years ago [8], was shown as leading to the best results [6,7]. Indeed, application of freeze-drying in ceramic processing offers many advantages, which are mainly related to preserving a high homogeneity or fine structure. Over the last twenty years, a number of patents involving freeze-drying as part of ceramic processing has been issued [patents 1-39]. Those concerning the preparation of perovskites [patents] describe processes based entirely on solution [7,30,34,40,41]. However, while all solution spray-freezing/freeze-drying methods lead to powders of superior quality, the overall process which consists of several following steps:
1 solution preparation, PA1 2 spray-freezing by atomizing the solution into a liquid nitrogen, PA1 3 vacuum (freeze)--drying of the spray-frozen material at pressures &lt;2 Pa, PA1 4 vacuum dehydration and partial nitrate decomposition at increased temperature (up to .about.380.degree. C.), and PA1 5 calcination at optimum temperature and time, is highly time intensive. PA1 1 Preparation of a slurry of lanthanide hydroxides, or other insoluble hydroxides formed by reaction of oxides (alkaline earth oxides) with water. This slurry may contain dissolved precursor components (for example strontium nitrate) which do not react with lanthanide hydroxides; PA1 2 Preparation of solution of remaining water soluble metal salt components, preferably nitrates; PA1 3 Combining the slurry of step one and solution of step two while stirring, or milling, and letting the mixture react; PA1 4 Fast freezing such as by spraying and freeze-drying the mixture of step three; PA1 5 Calcining the freeze-dried mixture of step four. PA1 1 solution preparation, PA1 2 spray-freezing by atomizing the solution into a liquid nitrogen, PA1 3 vacuum (freeze)--drying of the spray-frozen material at pressures &lt;2 Pa, PA1 4 vacuum dehydration and partial nitrate decomposition at increased temperature (up to .about.380.degree. C.) and PA1 5 calcination at optimum temperature and time.
Any shortcut in such processes leads to inferior quality powders. In particular, omission of step 4 will necessitate higher calcination (reaction) temperatures and time to achieve complete perovskite phase formation, and will result in coarser powders.
Other methods to produce fine perovskite powders include precipitation of the metal derived perovskite precursor components as hydroxides, carbonates, or as organic complexes with additional reagents. These methods may introduce undesirable impurities and usually require higher calcination temperatures to decompose all carbonates. When the method involves a combination of nitrates and organic compounds, there is a risk of uncontrolled explosion.
Gusman and Johnson (U.S. Pat. No. 4,975,415) describe a cryochemical method of preparing ultrafine particles of high purity superconductive oxides. This method makes use of soluble inorganic or organic salts of cations which may be metal, transition metal, rare earth or alkaline earth cations, preferably nitrates thereof. Oxides are mentioned as being usable in the invention, but are dissolved prior to use. Fine suspension or colloid of salts is contemplated, but such a suspension which is a very dilute mixture [concentration 0.015 to 0.15 M] is not allowed to react; it is immediately atomized. The suspension or colloid is contemplated only because of limited solubility of some precursors and to allow for potential incompatibility. Therefore, it is clear from this reference that solutions are highly preferred to suspensions, and there is no teaching of forming a slurry which is a heavy suspension containing a low amount of water in this reference. A high proportion of water renders necessary long freeze-drying times and, when additionally the salt content is high, the recovered solid is hygroscopic and requires a long time of heating to dehydrate the solid and to decompose the salts prior to calcination.
In another reference GB 2,193,713, Cabot Corp. describes a method of preparing perovskite type compounds, which comprises the steps of obtaining a slurry of hydrous titania into which is introduced a hot solution of barium hydroxide. A high temperature is maintained to obtain by this hydrothermal treatment divalent cation titanate(BaTiO.sub.3). Barium titanate may be doped with doping agents which include niobium, lanthanum, yttrium and nickel, manganese, iron and cobalt, added to the tetravalent titanium, either in the form of hydrous oxides or of soluble salts, such as nitrates. Dopants are introduced at a proportion of less than 5 mol % of barium titanate to provide a material which has a morphology similar to that of barium titanate. There is no teaching in this reference of the use of a lanthanide oxide as a primary component of a perovskite compound. There is further no teaching of the use of a lanthanide oxide and its conversion to an hydroxide in an aqueous slurry which would be a precursor for producing transition metal based perovskite.
Although the latter reference describes the formation of a slurry, there is no incentive of forming a slurry of trivalent lanthanum oxide (or other lanthanoids) and to use such a slurry in the production of lanthanide based perovskite powders.
There is no teaching in the Gusman and Johnson reference on how to replace a solution by a slurry and eliminate the use of the vacuum dehydration and partial nitrate decomposition step at an increased temperature prior to calcination. There is therefore room for improvement to substantially reduce the time of preparing such perovskite powders.